Myelinated axons contain beta-actin mRNA and ZBP-1 in periaxoplasmic ribosomal plaques and depend on cyclic AMP and F-actin integrity for in vitro translation.

Periaxoplasmic ribosomal plaques (PARPs) are periodic cortical ribosomal domains, which are likely focal sites of translational activity along myelinated fibers. b-actin mRNA, and its trans-acting binding factor, ‘zip binding protein-1’ (ZBP-1), were localized in PARP domains of axoplasmic whole-mounts isolated from goldfish Mauthner, rabbit and rat spinal nerve root fibers. Although RNA and ZBP-1 fluorophores showed co-distributions, co-localization signals of overlapped RNA and ZBP-1 pixels were skewed and asymmetric, suggesting possible transport-related RNPs. RT-PCR of b-actin mRNA confirmed its presence in RNA extracted from axoplasm of single Mauthner fibers. A metabolically active in vitro isolated Mauthner fiber system, which required cyclic AMP (cAMP) to activate translation, was developed in order to probe the importance of actin cytoskeletal integrity on cycloheximide-sensitive [35S]met/cys incorporation into axoplasm. Pre-treatment of fiber segments with cytochalasin D inhibited protein synthesis in axoplasm very significantly, while effects on the associated myelin sheath were modest and not significant. We conclude that activation of translation in axoplasm can be modulated by cAMP, and that the integrity of the actin cyoskeleton is essential for optimal translation. We suggest that mechanisms for targeting and localizing b-actin mRNA to PARP domains may be similar to those proposed for dendritic subdomains.